A liquid problem may contain the following cations: $\ce{Pb^{2+}}$, $\ce{Cr^{3+}}$, $\ce{Fe^{2+}}$, $\ce{Ca^{2+}}$, $\ce{Fe^{3+}}$, $\ce{Ag^{+}}$, $\ce{Au^{3+}}$, $\ce{Co^{2+}}$. I need to devise a scheme to separate these cations individually.

I have managed to prepare the following scheme:

enter image description here

But, I don't know how to separate the following cations: $\ce{Cr^{3+}}$, $\ce{Fe^{2+}}$, $\ce{Fe^{3+}}$, $\ce{Co^{2+}}$.

I thought I could use ammonia or sodium hydroxide as a reagent. If I use this reagent, all the species would be precipitated, if I am not mistaken, and therefore I could not separate them. Perhaps they could be separated by using solvents and then using identification reactions (for example, I know that Co(II) precipitates with 1-Nitrous-2-naphthol). But, at first sight, I see it as very complex.

  • 1
    $\begingroup$ Why don't you do systematic tests of cations from group 0 to group 6? $\endgroup$ – Zenix Jan 1 '20 at 15:56
  • $\begingroup$ I assume you're referring to the use of an analytical march. $\endgroup$ – aprendiendo-a-programar Jan 1 '20 at 15:58
  • $\begingroup$ Idk what analytical march is... Is it chemistry journal? I was talking about doing systematic wet tests (or you can also do flame test for $\ce{Ca^2+}$) for basic radicals. You can do flame tests for $\ce{Co^2+}$ or $\ce{Cr^3+}$ but you won't be able to differentiate between them. So we generally use it for tests of group 4 cations $\endgroup$ – Zenix Jan 1 '20 at 16:05
  • $\begingroup$ An analytical march is a technical and systematic process (a series of unitary operations), of identification of inorganic ions in a solution by means of chemical reactions in which the formation of complexes or salts of unique and characteristic color takes place. For example, the analytical march of the $\ce{H2S}$ $\endgroup$ – aprendiendo-a-programar Jan 1 '20 at 16:15
  • 1
    $\begingroup$ You mean a thing like that? bit.ly/2MNGINs $\endgroup$ – aprendiendo-a-programar Jan 1 '20 at 16:18

Well there is a pretty simple way, that you can do in any laboratory by standard reagents too. React the unknown mixture with $\ce{NH4OH + NH4Cl}$ (ammonium chloride is put as a standard to prevent the precipitation of Zn, Mn, Cu and Ni hydroxides, however, you don't have those in your sample, so you can proceed with only ammonium hydroxide too) now you will observe precipitate as

Green- confirms $\ce{Cr^{+3}}$

Reddish-brown- confirms $\ce{Fe^{+3}}$

White- confirms $\ce{Al^{+3}}$

(These are colours of the corresponding hydroxides)

Now if you have multiple salts, you carry out the following conformatory tests

For $\ce{Cr^{+3}}$, add a few dorps of strong oxidising agent in an acidified medium ($\ce{KMnO4}$/$\ce{H+}$ would do), this oxidises $\ce{Cr^{+3}}$ to $\ce{Cr2O7^{-2}}$ which gives the super famous chromyl chloride test.

For $\ce{Fe^{+3}}$, add a few drops of $\ce{CH3COONa}$ and you will initially observe a dark-red solution of $\ce{Fe3(OH)2(CH3COO)6}$, which on heating precipitates as blood red $\ce{Fe(OH)2CH3COO}$

For $\ce{Fe^{+2}}$ simply add KI, very distinct black $\ce{FeI2}$ precipitates out

For $\ce{Al^{+3}}$ add ammonium phosphate, you will see a cloudy white precipitate

You can read more here, Vogel is pretty much the most solid and trustworthy book for practical inorganic chemistry You are welcome!

  • $\begingroup$ I had thought of using sodium hydroxide. But, if the liquid sample is made up of all or most of those cations, a mixture of precipitates will be obtained. And, in that case, I won't be able to distinguish which species it is. Obviously, if when I add the hydroxide, I get only one green precipitate, I know that the mixture only contains chromium and the other cations are not part of it. $\endgroup$ – aprendiendo-a-programar Jan 1 '20 at 16:26
  • $\begingroup$ well, standard tests get hard for multiple salt compounds. You would have to carry out specific confirmatory tests, and you cannot do this easily in a small laboratory. So it'll be a pretty tough task for say an undergrad level student. $\endgroup$ – Haha Hahaha Jan 1 '20 at 16:32
  • $\begingroup$ Should I send you link, where you can give a deep study to the topic? There are confirmatory tests for each radical, you can accurately find if any particular cation is there or not? $\endgroup$ – Zenix Jan 1 '20 at 16:36
  • $\begingroup$ Yes Zenix that would be nice, thanks a ton! $\endgroup$ – Haha Hahaha Jan 1 '20 at 16:42
  • $\begingroup$ You could re-edit and add it in my answer too, that would be helpful for the entire community! $\endgroup$ – Haha Hahaha Jan 1 '20 at 16:43

If you add NaF, it will make a complex with Fe(III) that will not react any more with any reagent later on.
Cobalt may be recognized by adding solid NH4CNS and some amyl alcohol. If there is Cobalt in the solution, the alcohol phase will get blue. Iron must be previously eliminated by adding a little solid potassium or sodium fluoride. Fe(III) may be recognized by adding a few drops of a dilute NH4CNS solution in the solution. A red color proves the presence of Fe(III). Cr(III) may be recognized by bleach (NaOCl solution). if heating the solution with bleach produces a yellow solution, it proves that Cr was present initially, as this yellow color is due to the chromate ion. Of course, it will be necessary to filtrate the solution, as all the other metallic ions will make a dark precipitate of mixed hydroxides after adding bleach.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.